The present invention relates to a method as defined in the preamble of claim 1 for spraying an aqueous liquid into the air intake duct of a turbocharged piston engine for humidifying the intake air to reduce nitrogen oxide emissions.
The invention also relates to a spraying apparatus as defined in claim 3 for humidifying the intake air of a turbocharged piston engine to reduce nitrogen oxide emissions, said apparatus comprising at least one nozzle for spraying an aqueous liquid into the air intake duct.
The invention thus concerns especially a method and apparatus for supplying water into the intake air of a turbocharged piston engine to reduce nitrogen oxide emissions (NOx). At the high combustion temperatures, the combustion process in the cylinder of a piston engine produces nitrogen oxides, which are emitted together with the exhaust gases into the atmosphere. Because of the harmful climatic effects of nitrogen oxide emissions, efforts are undertaken to minimize their production.
As is known, adding water to the combustion process in the form of either water vapor or water droplets reduces the generation of nitrogen oxides. This phenomenon is based on a cooling effect. When the water sprayed into the cylinder is evaporated, it reduces the temperature of the air in the cylinder while at the same time reducing the pressure. The pressure drop has an adverse effect on the efficiency, although the decrease of pressure and temperature has a favorable effect on the formation of nitrogen oxides. When the water is supplied in the form of droplets together with the intake air, some of it is additionally wasted during the scavenging period and water consumption is increased. When air saturated with water vapor is supplied into the cylinder, the thermal capacity of the filling gas is increased and the gas has a substantially greater effect of reducing the temperatures of the combustion process than does dry air. The effect of reducing the combustion temperatures increases with the water vapor concentration, yet without producing an undesirable effect on efficiency. Since an increase in the temperature of the gas supplied into the cylinder also augments the generation of nitrogen oxides as well as the consumption of water, it is desirable to keep the gas temperature as low as possible, yet high enough to ensure that the gas supplied into the cylinder contains an amount of water vapor sufficient for the reduction of nitrogen oxides.
An apparatus for vaporizing a desired amount of water is disclosed in patents U.S. Pat. No. 5,758,606 and U.S. Pat. No. 6,196,165. A drawback with this apparatus is that the device mounted between the turbocharger and the cylinder increases the cubic volume of the air intake ductwork, which has a considerable effect on the power output of the engine. The power output is dependent on the cubic volume after the turbocharger because during power increase or decrease the air pressure produced by the turbocharger increases the density of the air and the amount of gas entering the cylinder. If the cubic volume between the turbocharger and the cylinder is increased, it will take considerably longer before the amount of air produced by the turbocharger brings the pressure to the desired level and the power generated by the engine increases. Another drawback with the apparatus is that the heated water used for vaporization and flushed over the evaporation surfaces has the effect of increasing the temperature of the air. The device is unable to make use of the cooling effect produced in connection with the vaporization of the water, but the gas output from the device is at a relatively high temperature, so the amount of water vapor required for the reduction of nitrogen oxides and therefore also the water consumption are increased considerably.
Specification WO98/10185 again discloses an apparatus in which the air produced by a turbocharger and the pressure of this air are utilized in the injection of water for humidifying the air supplied to the turbocharger. A drawback with this system is the relatively low temperature of the supply air, which is why the amount of water vapor evaporated into the air remains small, and thus no significant nitrogen oxide reducing effect is achieved. Another drawback is that when the amount of water is increased, the water droplets can evaporate after the air has reached a saturated state, with the result that the water droplets drift into the turbocharger and cause wear of the turbocharger vanes through droplet erosion. From a thermodynamical viewpoint, the drifting of droplets into the turbocharger is desirable as it reduces the work performed by the turbocharger, increasing the pressure of the pressurized air produced at the output and simultaneously reducing its temperature. In practice, however, a turbocharger rotating at a very high speed—about 50,000–100,000 rpm—has proved to be very sensitive to droplet erosion as referred to above.
The object of the present invention is to achieve a spraying system for supplying water mist into the air intake ductwork of especially a piston engine and allowing the drawbacks of prior-art to be avoided. An additional object of the invention is to achieve a method and apparatus that will enable efficient humidification of intake air.
The method of the invention is mainly characterized in that, in a first stage in the method, the intake air is heated by a heat exchanger element before the turbocharger and water mist is injected into the air intake duct after the first heating stage through at least one first nozzle, that in a second stage the intake air is compressed by the turbocharger, causing its temperature to rise, and water mist is injected into the air intake duct through at least one second nozzle after the second stage.
The apparatus of the invention is mainly characterized in that the apparatus comprises at least one heating element for heating the intake air before the turbocharger and at least one first nozzle for injecting an aqueous liquid mist into the air intake duct after the heating element.
The apparatus of the invention is further characterized by what is stated  in claims 4–10.
The solution of the invention has numerous significant advantages. The apparatus is connected directly to the structures of the air intake duct and it produces a fine mist directly without using any extra chambers or other containers, it is able to make full use of the heat quantity required for the vaporization of the water, cooling the intake air at each spray injection point to a temperature close to the wet bulb temperature (or adiabatic saturation temperature, which in the case of a water-air mixture is practically the same thing), i.e. to the temperature to which the air temperature can be reduced by vaporization of water. As connecting the apparatus of the invention to a turbocharged engine does not involve any changes in the cubic volume of the air intake system, it has no adverse effect on the power output of the engine, either.
By the method of the invention, very good and efficient humidification of intake air is achieved. The heating of intake air used in the method can be implemented using a relatively economic construction as it does not have to withstand the high pressure after the turbocharger. The efficiency of the turbocharger is increased because the mass flow passing through it is larger than before. In addition, the engine's response time for power boost is accelerated because the turbocharger duct volume is reduced.
Another advantage of the apparatus of the invention is that the humidity of the intake air can be increased stepwise after each heat supply point, yet before the last heat supply point, which can be used as a water evaporation surface, thus controlling the humidity of the gas fed into the cylinder and therefore the formation of nitrogen oxides within desired limits.